Slowing down an induction motor with 555? or microcontroller?

[jbay]

I have a Dayton 60 rpm gearmotor that swivels a long metal bar as it turns. Connected to the bar are several dissolved oxygen probes. I need to slow the motor down so that it will not rotate these probes so quickly.

Is there a way to reduce the motor speed using a microcontroller? or to just have it turn on for about 2 seconds every 30 seconds using a 555 timer and relay? Normally, another gear would do the job, but I already built this contraption and like the challenge of solving the problem with an electric circuit.

My background is in environmental engineering; but, I'm trying to pick up electronic circuits as a hobby and would like to create a circuit to slow this motor down.

Thanks for your help.

 

[Boncuk]

Hi jbay,

switching an AC motor at intervals with a full stop between switching cycles (120 cycles per hour) might lead to motor overheating.

If its rated power is not too high (100 - 150W) you might use a filament type lamp dimmer to control speed.

Boncuk

 

[dknguyen]

THe speed of an induction motor is dependent on the frequency of the input waveform. Do not try to PWM it like a DC brushed or brushless motor to slow the motor down. The speeds of those motors is dependent on average voltage seen which is what PWM does and is the limit of what a 555 timer is capable of. Not a microcontroller though, but the microcontroller is not the problem in this case- it's all the power circuitry required to pull off the solutions I am about to talk about is.

Since the speed of induction motors is dependent on the voltage and frequency of the AC signal, you need a drive that can produce a wide range of voltages and more importantly frequencies. These called variable frequency drives and are the most complex of conventional motor drives. Many (if not most) do incorporate microcontrollers. Then there are even more complicated weird kinds that can convert AC of one voltage and frequency directly into AC of another voltage and frequency...really...strange things.

The best you can do with a 555 timer or a microcontroller (without extensive power circuitry and coding) is turning the motor on and off so it alternates between full speed and no speed which is not advisable unless you are talking about infrequent switchings (like on the order of once a minute or more).

Easiest thing to do would be to change the gear ratio. If you want an electrical challenge, then replace the motor with a brushed DC motor. It sounds like you only need unidirectional speed control so that means a single-transistor motor drieer being controlled by a 555 timer wired up as a PWM generator or a microcontroller doing the same job (and maybe a gate driver to help the 555 timer or MCU drive the transistor. THis should be completely within your capability as is a rather nice starter project.

Beyond that are things like H-bridges for bidirecitonal speed control of brushed DC motors. Then there are trapezoidal synchronous motor drivers followed by sinusoidal synchronous motor drivers (which with some modification in the control loop are basically what is required for induction motors.

Naming Convention FIasco:Induction and synchronous motors both contain no brushes, but normally when someone says brushless motor they are talking about a specific class of synchronous motor that uses permanent magnets and is designed to run off trapesoidal waveforms. And even those these are synchronous motors, many times when someone says synchronous motor they are referring to a specific class of synchronous motors that use field windings instead of permanent magnets and are designed to run off sinusoidal waveforms tend to be called synchronous motors.

 

[Jaguarjoe]

2 seconds out of thirty is not a bad duty cycle and a bar with some probes on it is not a big load. A microcontroller driving a solid state relay would be very simple and inexpensive to implement. This would be a good way to learn uC's and here is a good place to start:



Join the forum and the good guys over there will be more than happy to help you.

 

[crutschow]

If you only need to turn the motor on for 2 seconds out of 30 then I would suggest a timer (perhaps built with a couple 555's) to do that, as the simplest solution. I don't see how that could cause any motor over-heating. The timer could switch a solid-state AC relay to control the motor.

Here's a timer kit that may also do the required timing (mode 7). Edit: Or perhaps this.

 

[Boncuk]

If you only need to turn the motor on for 2 seconds out of 30 then I would suggest a timer (perhaps built with a couple 555's) to do that, as the simplest solution. I don't see how that could cause any motor over-heating. The timer could switch a solid-state AC relay to control the motor.

Hi Carl,

a three phase motor is also an induction motor. A customer of mine had the expensive hobby to shut down and start one fan depending on the necessary air flow.

He did it at irregular intervals exceeding 20 starting cycles per hour what caused the motor to overheat and go up in flames.

Induction motors draw four to six times the nominal current at start up what causes overheat.

This motor will cycle through 120 starts per hour which will most probably kill the motor.

I designed a circuit for the fans allowing to start immediately with a delayed shut down of 10 min while the blade angle was reduced to almost zero air flow, resulting in a safe temperature for the fan motor (maximum of 6 cycles per hour).

Regards

Hans

 

[crutschow]

a three phase motor is also an induction motor. A customer of mine had the expensive hobby to shut down and start one fan depending on the necessary air flow.

He did it at irregular intervals exceeding 20 starting cycles per hour what caused the motor to overheat and go up in flames.

Induction motors draw four to six times the nominal current at start up what causes overheat.

This motor will cycle through 120 starts per hour which will most probably kill the motor.

Well, a fan motor has a high inertial load, which makes for a long startup time and extended startup power dissipation, so I can see how many starts and stops could burn up the fan motor.

But a gear motor has a relatively small equivalent inertia load (since it's gear reduction output shaft is just connected to a bar) so it should have a short startup current duration and corresponding small startup power dissipation. Thus I think that it could tolerate one startup every 30s with 28s to cool between without overheating. However, I have no solid evidence to support that opinion.